Deleted objects still exists
Objects are "containers" for data. Mesh objects contain mesh data, curve objects contain curve data, etc. When you deleted the curve, you deleted the container, not the data that it contained. So, in your code your operating on the first curve in the curve data block, which would be the original curve.

Some options you have to resolve this are:

a. Delete the curve from the curve data block
 b. Find the curve that the curve object contains, and operate on it.

I would do the later:

curve = bpy.data.objects["curve object's name"].data
spline = curve.splines[0]